The invention relates to gas-insulated buried metal-clad electricity lines. Such a line comprises a cylindrical metal case filled with a dielectric gas under pressure, generally sulfur hexafluoride (SF6) or a mixture of sulfur hexafluoride and nitrogen, and an electrical conductor is disposed therein. In use, the case of the line is buried in the ground at a depth of two or more meters.
The invention relates more particularly to a method of measuring the density of the dielectric gas under pressure inside the case of a buried metal-clad line.
In general, measuring the density of the gas contained inside the case of a buried metal-clad line consists in measuring the pressure P and the temperature T of the gas and in compensating the pressure measurement as a function of the temperature measurement using constant density curves D1, D2, and D3 for the gas, as shown in FIG. 1. That compensation can be performed either in a data acquisition module receiving the pressure and temperature values from two distinct sensors, or else directly in the housing of the pressure sensor which then becomes a density sensor. In the latter case, temperature is measured by the sensor and the output signal from the sensor (e.g. a signal at the industrial 4 mA-20 mA standard) is representative of the measured density.
A large error in determining the density of the gas is due to the fact that the temperature sensor, regardless of whether or not it is integrated in the pressure sensor, is mounted on the outside surface of the case of the line such that the sensor measures the temperature of the gas only in a zone close to the surface of the case. Unfortunately, this temperature measurement is generally not equal to the mean temperature of the gas inside the case, so it follows that the density as measured suffers from an error of the order of several percent. By way of example, as shown in FIG. 2, the temperature TS of the case can be several degrees Celsius (typically 5xc2x0 C.) lower than the mean temperature of the gas TG inside the case because a nominal current is being carried by the conductor of the line. As a result, the density D3 as determined from the measured pressure P and the measured temperature TS can be greater than the real density D2 of the gas. This error penalizes protection of the metal-clad line.
To mitigate that drawback, it is known to compensate the measured density as a function of the current carried by the line conductor on the assumption that the greater the current carried by the line conductor, the greater the error in the measured density of the insulating gas. Nevertheless, that solution is expensive to implement since it requires an additional physical magnitude to be measured, namely the current. Unfortunately, the current carried by the metal-clad line can be measured only at an inlet stage, so it is necessary to disseminate this measured information over all of the density sensors mounted along the metal-clad line, and in practice that is complex and expensive.
The object of the invention is thus to propose a method of measuring the density of a dielectric gas under pressure in a buried metal-clad electricity line that does not present the above drawback.
The Applicant has observed that with buried metal-clad lines, there is a direct link between the current carried by the line conductor and the temperature difference between the surface of the case of the line and the gas inside said case due to the fact that the ambient temperature around the case varies little and, in particular, that for a metal-clad line that is buried the envelope is not subject to the effects of solar radiation.
More particularly, the invention provides a method of measuring the density of a dielectric gas under pressure in a buried metal-clad electricity line, the method consisting in measuring the pressure and the temperature of the dielectric gas by means of a sensor mounted on the case of the line, and in compensating the pressure measurement as a function of the temperature measurement using constant density curves for the dielectric gas, wherein the constant density curves are corrected on the basis of a curve representative of the difference between the temperature at the surface of the case of the line and the mean temperature of the gas inside the line for increasing amounts of current carried by the line conductor.
The invention also provides a sensor for implementing the method of the invention.